1. Field of the Invention
This invention relates to a process for producing an anti-reflective protective coating for use as a photographic filter, eyeglass lenses, optical filters or the like, by vacuum sputtering.
2. Description of the Prior Art
The conventional anti-reflection coating layers, as illustrated in FIG. 1, that are generally used comprise a coating layer 2 of 3/4 in thickness of the predetermined wavelength of a medium refractive index material, such as CeF.sub.3 or Al.sub.2 O.sub.3, coated on the glass 1 having a refractive index of 1.52; layer 3 of a high refractive index material ZrO.sub.2 of .lambda./2 in thickness coated on layer 2; and a layer 4 of the low refractive index material MgF.sub.2 of .lambda./4 in thickness coated on layer 3, all of these layers applied by a vacuum sputtering method. The curve of reflection ratio of this conventional coated glass is shown as dashed line a in FIG. 3 and the predetermined wavelength, that is the reflection ratio at 500m microns, is about 1% which is considerably high. This reflection ratio changes with the degree of the non-uniformity or nonunity of the high refractive index material. As its non-uniformity becomes larger, reflection of the predetermined wavelength increases.
In the multiple coating layers on the glass, conventionally ZrO.sub.2 has been considered to be the best material among the high refractive index materials, with respect to light absorption of the coating, strength of the adhesivity of the coating to the glass, and the stability of the refractive index. However, when the ZrO.sub.2 layer is vacuum sputtered onto the glass, and as the coating thickness becomes thicker, the density of the molecules become smaller and the refractive index becomes lower, thereby providing a larger non-uniformity in the coating, thus sacrificing the anti-reflection properties of the coated glass.